Wild Horse Island is an island in Flathead Lake, a natural, fresh water lake in Montana. Flathead Lake is the largest natural, freshwater lake west of the Great Lakes. Formed by glacial action, it is 200 square miles in area. Wild Horse Island is found in the middle part of the lake, about a mile from the western shore, which has several small towns on it.
Wild Horse Island is about 3 miles wide by 2 miles long at its maximum points, and is a little less than three square miles in area. It reaches about 800 feet above the level of the lake at its highest point. It is the home of a state park, and its area, while small, is large enough to maintain populations of wild animals, including Big Horn Sheep, Deer and the titular mustangs. It probably doesn't have any resident apex predators, although puma may visit. Although it is a state park, it has no developed facilities for visitors.
It does, however, have several dozen permanent homes. Without electricity, and reachable only by boats at their private docks, these are only vacation homes. According to Zillow, a one-bedroom, one-bathroom home on the island goes for over a half million dollars. Although the island has no permanent residents, and has no services or commercial establishments, and its few dozen vacation homes barely constitute a major settlement, it is, as far as I can tell, the most heavily populated island without bridge access, between the Great Lakes and the Pacific Coast. Someone who finds themselves living, however temporarily, on Wild Horse Island, will be in a state of isolation from the rest of the continent that is difficult to find anymore, and is separated from the world in a way that people a mile away, on the shore of the lake, are not.